This invention relates generally to the CRT of a projection-type television receiver system, and is particularly directed to protecting the CRT of a projection-type television receiver from damage to its phosphor-coated faceplate.
In general, a video display utilizing a cathode ray tube (CRT) such as a television receiver includes a low voltage power supply for driving the scanning and receiver circuitry and a high voltage power supply for energizing the accelerating grids of the CRT. CRT electron beam current intensity is controlled by the relative potentials on the cathode and control grid electrodes with the cathode being coupled directly to the video input signal. The various electrode grids in the CRT perform functions such as brightness control, picture focusing and contrast and background selection.
Electron beam deflection circuitry provides for the raster scanning of the CRT's faceplate by the electron beam. The electron beam is scanned horizontally across the CRT's faceplate and sequentially deflected vertically in accomplishing the aforementioned raster scan. Damage to the phosphor coated faceplate of the CRT may result if the high voltage is applied to the various electron beam control grids while the deflection voltage is removed from the CRT. Loss of electron beam deflection signals may be due to failure of the horizontal and/or vertical deflection circuits, the absence, either intermittent or permanent, of a respective sweep drive input signal, or any number of related television receiver system malfunctions. If only one of the deflection signals is removed from the television receiver, the CRT's faceplate will be burned along a single straight line parallel to the sweep direction of the deflection system still in operation. If neither deflection signal is provided to the CRT, a burned and permanently damaged spot will occur on the viewing screen at the electron beam free-fall center of the CRT.
Phosphor burn is a greater potential problem in a CRT as utilized in a projection receiver than in a conventional television receiver. This is due to the unique configuration and operation of a projection television receiver (CRT). For example, a projection television system suffers from inherent limitations in the brightness of the reproduced image on the projection screen. These operating limitations are due to light losses in the image projection lens system, absorption and dispersion of the light energy containing image information incident upon the projection screen, reflection losses if a mirror system is used, and the large size of the projection screen relative to the total surface area of the projection television system's CRT, or CRT's.
In order to overcome the aforementioned limitations, various approaches have been implemented. The most common approach to increasing image intensity, or brightness, is to increase the inter-electrode voltages which accelerate the electrons toward the phosphor-coated faceplate. This has resulted in an increased danger of permanently damaging the CRT's screen by means of a continuous beam of undeflected, energetic electrons impinging on the same spot, or line, when deflection voltages are removed.
A shadow mask is typically utilized in a conventional color television receiver CRT. However, in order to increase the image intensity in a projection television receiver the shadow mask has been removed from the CRT and multiple CRT arrays, with each CRT emitting a light beam of one of the three different primary colors of red, green and blue, are commonly utilized. Thus, the energy absorption provided by a shadow mask in a conventional color CRT is not available in a projection television CRT, resulting in a greater potential for unregulated electron beam damage therein.
One approach to protecting the projection television system's CRT faceplate is disclosed in Quasar Electronics Company Service Manual for Projection Chassis ANVDTS-800 published in 1978 by Matsushita Electric Corporation of America. This system utilizes a deflection yoke for each of the primary color electron beams in the CRTs. Under normal operating conditions the outputs of the three transformers are provided in a balanced manner to two additional transformers. During abnormal operation of any of the three deflection yokes, the outputs of the two additional transformers become unbalanced producing a resultant output pulse used to drive CRT shutdown circuitry. A third additional transformer is used to detect the failure of all three deflection yokes and to generate an output signal for CRT shutdown in response thereto.
U.S. Pat. No. 4,297,619 to Kiteley discloses a system for preventing CRT phosphor burn in a multiple CRT projection television system in which only a single transformer in each of the vertical and horizontal deflection systems is utilized to detect the absence of a deflection yoke signal and to provide a signal to CRT shutdown circuitry in response thereto. Each of the transformers in the vertical and horizontal deflection systems is coupled to the three respective deflection yokes in the three CRTs. The absence of any one electron beam. deflection signal or all three deflection signals results in activation of CRT shutdown circuitry.
While the aforementioned approaches provide for electron beam termination following failure of horizontal deflection yokes, they fail to account for situations which do not result in deflection yoke circuitry failure, but which may also for only a short period render the CRT susceptible to phosphor burn. This may occur in the case of an intermittent horizontal drive signal provided to a deflection system causing horizontal sweep to collapse and therefore show a single vertical line on the CRT's faceplate for several milliseconds. Because of the unique operating characteristics of a projection television CRT, this occurrence will generally result in phosphor burn in the CRT. In addition, prior art projection television CRT protection systems do not address the problem of total horizontal deflection system failure. The present invention is intended to overcome this limitation in a projection television receiver system and to offer CRT protection in an area not heretofore addressed by prior art systems.